Seeking fix for Juki LU-2810 upper thread breaking on hook/bobbin assembly

[LMC]

Hello,

Seeking input on what might be going on with a new and new to me Juki LU-2810N where the upper thread is breaking. This happens under med to medium-high upper thread tension especially if I'm working with multiple layers of a heavier fabric such as exterior vinyls. The servo motor is 750 w so there's no issues with motor power observed.

I've tried all the regular adjustments with improvement at lower upper tension. However, if I'm working with multiple layers of vinyl, I have not been able to resolve the issue through regular machine adjustments: upper thread tension (2 so equally adjusted), bobbin tension, uptake spring tension, uptake spring location (sweep).

I make exterior canvas and upholstery and I generally use SolarFixx PTFE thread in a 138 weight equivalent for its best in class UV radiation resistance properties.  It is a slippery thread and stronger than the equivalent weight polyester outdoor thread.

The photos below shows the issue. Described in words:

As the upper thread is picked up by the hook it is swept over the top of the bobbin case/hook assembly.

As the thread on the hook approaches the lower right quadrant the thread becomes loose from the hook or hops off the hook.

Then, as the upper thread comes under tension it catches on the opening in the bobbin case that is used for threading the bobbin to the needle plate.

As the upper thread comes under more tension, as the needle lifts, it gets pulled further into the bobbin case opening.

Meanwhile, the hook is continuing to rotate. As the hook rotates further clockwise it makes contact with the thread stuck in the bobbin case and then the upper thread is cut or broken by the rotating hook.

If the thread was staying attached to the hook a little bit farther through the hook rotation cycle the upper thread loop would likely clear the bobbin slot where it is getting caught and the issue would be resolved??

Is this a hook timing issue or is something else the cause of the issue here?

I'm deep into many, many hours of trouble shooting this issue... Partly because I'm new to this machine.

What are the recommended next steps to fix this issue? 

Edited May 1 by LMC

[kgg]

1 hour ago, LMC said:

Seeking input on what might be going on with a new and new to me Juki LU-2810N where the upper thread is breaking.

You should refer to your manual as the threading of the bobbin is not correct Section 3.3 and 3.4 will detail how to thread the bobbin case.

kgg

[LMC]

Hi KGG, I really appreciated the reply about bobbin threading!

I'll recheck the bobbin threading now as I'm at my machine.  FWIW, the bobbin / lower thread has not been an issue (as far as I know). 

There are multiple versions of this machine (which you likely know so adding here for anyone who might not know). 

My version, LU-2810N is the fully manual, external motor, Singer style foot base model. No thread trimmer, automatic back-tack, etc. that the automatic models have.

I did notice in my efforts to address the upper thread breakage problem that manual indicates for the automatic versions of the 2810 family of machines that the lower thread exits where the bobbin "latch" lays over the bobbin.

However, my version has a slightly different thread path indicated in the manual and the thread does not exit at the bobbin latch. (I suspect the bobbin latch exit path has something to do with the versions which have a thread trimmer which my machine does not have).

I've attached the manual pages to this thread for additional input and for any future reference that might help someone else. Or in case I've misunderstood the somewhat sparse information in the manual.

In any (bobbin) case, I'll try the bobbin latch thread path and see if this resolves the upper thread issue.  The machine vendor I purchased from also indicated to use the threading path I've been using however they could be mistaken.

I'll also see if I can upload a video of the issue here for inspection by anyone interested.

And, I'll report back on the alternate bobbin threading suggested, hopefully with positive news.

[LMC]

I am not finding any way to thread the bobbin with a clean run of the lower thread under the bobbin latch (location 2 in the manual section on bobbin winding for automatic 2810 machines). 

Possibly there is a different bobbin case for the automatic machines which allows the thread to cleanly / smoothly pass under the bobbin latch?

Close up photos attached.

I believe I am threading the lower thread and bobbin correctly for this version of the 2810 without going through the bobbin latch (but maybe I'm missing something).

Could the upper thread getting pulled into the bobbin case slot behavior be a result of some intrinsic property of the SolarFixx thread which is a fairly stiff thread compared to the same weight of polyester thread?

Or maybe with multiple layers of vinyl a larger gauge needle is needed due the increased friction on the thread passing through many layers?

Or maybe it is a slight timing adjustment which is the one thing I haven't tried myself although I did have the machine vendor recheck the timing (before I got deeper into sewing machine maintenance and repair research).

 

[LMC]

Note: if the video here buffers on first watch try and run it again after it fully runs the frist time and it may run smoothly without any buffering as it will be temporarily saved on your browser's memory.

 

I'm hand rolling the flywheel here. The same behavior happens at slow motor speed (50 rpm).  At higher motor speed I'll get some stitched in correctly and then it will start to fail with loops on the underside or the upper thread will break.

Here's what happens when I try to sew multiple layers of vinyl.  Upper thread catches on the bobbin case at the slot. Often it gets sucked into the slot. This happens regardless of upper thread tension. I either get a mess of loops on the underside or the upper thread breaks when it gets snapped or cut by the hook after getting caught. 

On fewer layers of canvas or vinyl I'm getting good stitches.

Maybe I'm exceed some machine spec for material handling but that doesn't seem quite right to me.

Edited May 1 by LMC
Added note about attached video buffering

[kgg]

2 hours ago, LMC said:

On fewer layers of canvas or vinyl I'm getting good stitches.

A couple of questions:

1. what size of needle are you using??

2. how thick is the material that is causing the problem??

3. is the tap on the bobbin case fully seated in the needle plate???

I am guessing maybe the needle to hook timing may need to be changed slightly. 

kgg

 

Edited May 2 by kgg

[friquant]

I had that problem on my pfaff 545: https://leatherworker.net/forum/topic/123071-pfaff-545-alignment-of-bobbin-case-cap-base/

I have not had a bobbin of your style yet, but I conjecture the same thing is happening. On the pfaff 545 the two pieces that meet up are separate pieces, one is part of the bobbin case and one is part of the bobbin case cap. And the one that sticks out the farthest needs to be the one that's upstream, otherwise the thread will get caught on the downstream one. For me the resolution was to remove some material from the bobbin case (the downstream piece) so the thread would always pass by without getting drawn in between the two. (See the photos in the first and last posts by me to see the difference.)

In your case you have a groove in a single piece of metal. You can still alter the shape on the leeward side of the groove, but I recommend wrapping some tape around the upwind side of the groove so you don't accidentally knick it. Then take a diamond file to the area just downstream of the groove. File it back enough so that it has a nice "underbite" such that the thread cannot catch on the downstream edge of the groove. Once you've got the geometry correct, polish it with some 320 grit and then 600 grit paper to make sure it is gentle on your thread.

[Wizcrafts]

It looks like the top thread may be too tight at pickoff time. Tight thread is likely to get caught on channels in the bobbin case, as yours does. First, try advancing the hook slightly. If the timing is lagging, the thread will be tight going around the bobbin case/shuttle. Advancing it can take some of the tautness out of the equation.

If the hook timing doesn't help, perhaps you need some more slack in the top thread. You can add it via the check spring assembly. If your machine's check spring mounted in a disk with a cuved slot and screw, loosen the screw in the slot and turn the assembly counterclockwise all the way, then tighten down the set screw. If the disk doesn't want to turn, loosen the nut on the shaft.

If the overall tension is too high, reduce the bobbin spring tension, then the top tension to center the knots with less overall thread tension.

As for the correct needle vs thread sizes, consult the thread and needle chart on Toledo Industrial Sewing Machines website.

[LMC]

Hi KGG appreciate your replies! Answers to your questions:

1. what size of needle are you using??

135x17x21 & 23 Groß Beckert San 5 or DP (?) 

 

2. how thick is the material that is causing the problem??

The upper thread catch happens more often with thicker/heavier/tougher materials. With four five six layers of marine acrylic canvas fabric (e.g. Sunbrella) I can adjust the machine enough with the regular adjustments to dial out the issue.  However, I recently installed a Kinedyne for more low and slow functionality (replacing a 550 W Consew servo motor). With the new 50 rpm and 45 mm drive pulley sheave the thread catching on the case became more likely than with the Consew motor which had a larger drive sheave (65 mm) and a min speed of 200 rpm.

The upper thread catch is most apparent with 8 or 10 layers of vinyl. Maybe it's 1/2” or 5/8".

Or less of a tough fabric.  With the Consew 550 watt this was also happening.  

Both motors where able to drive the needle without shudder or cutout or tripping the machine breakaway clutch.

3. is the tap on the bobbin case fully seated in the needle plate???

I'm not clear on the question here. The bobbin is the one provided by the sewing machine vendor. It is the correct size and it fits in the case correctly as far as I can tell. This design has a small light spring that is permanently installed in the bobbin case. The bobbin floats on the spring and is sandwiched between the spring under and the bobbin case latch above. The bobbin spins freely.

This issue happens equally with all my new bobbins including the Juki supplied bobbin.

I am guessing maybe the needle to hook timing may need to be changed slightly.

Definitely could be.

[LMC]

Hi Friquant,

Appreciate the mechanical alteration suggestion. And the link to your documentation here on the forums. I definitely would not have found that easily. So many kinds of  machines!

The mechanical alteration option was definitely one of the options on my mind as a possible solution.

I figured, I'd reach out here first for a sanity check on other solutions and input before taking on a mechanical mod.

I'll read through your case modifications topic on your solution tonight.